Continuous flow reactor having a plurality of alternating bends

ABSTRACT

A curved tubular flow reactor designed for carrying out chemical reactions continuously, for preparing mixtures and as a liquid-phase heat exchanger comprises a plurality of successive bends having alternating directions of curvature.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a curved, tubular flow reactor havingan essentially circular or elliptical cross section, to the use of thisapparatus for carrying out chemical reactions continuously and to aprocess for continuous polymerization using the novel apparatus.

2. Description of the Background

When chemical reactions are carried out on an industrial scale,continuous-reaction engineering often provides advantages over reactionsbeing run discontinuously. This is particularly true in those caseswhere large volumes have to be coped with and discontinuous-reactionengineering is likely to result in uneven product quality from batch tobatch. However, it is often difficult for continuous-reactionengineering to be put into practice. One of the fields to which this isapplied is emulsion polymerization and suspension polymerization ofethylenically unsaturated monomers.

There has been no lack of attempts at the practical implementation ofcontinuous-reaction engineering in this field. The first patents oncontinuous emulsion polymerization were filed as early as 1937 and 1938by I. G. Farbenindustrie (GB 517,951 and FR 847,151). By now numerousapproaches to continuous emulsion polymerization are known. Describedmost frequently is the use of a continuous stirred-tank reactor, anoverview being provided, for example, by Encyclopedia of Polymer Scienceand Engineering, 1986, Vol. 6, pp. 11 to 18. Emulsion polymerization insuch reactors results in a wide particle size distribution, and theestablishment of a steady state takes a relatively long time or failsaltogether. Moreover, owing to the shear-force action of the stirrer,there is an increased tendency for the polymer to coagulate and then alltoo readily to settle on the stirrer or on reactor internals.

A particular refinement of the abovementioned stirred reactors isdescribed in DE-A-1,071,341. The reactor, a cylindrical tube, isequipped with disks seated on a shaft, which result in Taylor ringsbeing formed when the stirrer is set in rotation. The main effect ofthis is to improve mixing perpendicular to the flow direction, therebyimproving the space-time yield. Reactors of this type always comprisemoving parts. This has the drawback that constructional measures arerequired for supporting and sealing the shaft. Moreover, accretions anddeposits of polymer are formed owing to gas bubbles present or beingformed in the system, which collect preferentially at the shaft and inlow-flow zones and there trigger coagulation of the polymer. Finally,the rotating disks give rise to shear forces which likewise lead tocoagulation of the polymer.

A further improvement is represented by reactors which comprise twoconcentric cylinders, the inner and/or outer cylinder being set inrotation. Depending on the speed of rotation and the flow velocity,various stable flow patterns are formed which can cover a range withlaminar Couette low at one end, the formation of Taylor ring vortices inbetween, up to turbulent vortex flow. Polymer International 30 (1993),203-206 describes a continuous Couette-Taylor vortex reactor and its usefor continuous emulsion polymerization) resulting in latex particleshaving a relatively narrow particle size distribution. ChemicalEngineering Science, Volume 50, pp. 1409-1416 (1995) describes theemulsion polymerization of styrene in a continuous Taylor vortex-flowreactor. Since reactors of this type do not employ a conventionalagitator, they are particularly suitable for preparing polymerdispersions which have a particularly pronounced tendency to coagulateunder the influence of shear forces. Moreover, similar drawbacks ariseas in the reactor described in DE-A-1 071 341.

An alternative to the stirred reactors is the use of pulsed, packedcolumns as described in Chem. Eng. Sci. Vol. 47, 2603-2608 (1992) andEP-A-336 469. It is thus possible for styrene or vinyl acetate to bepolymerized continuously. However, columns of this type will veryreadily foul and plug.

A further alternative to the stirred-tank reactors and stirred tubularreactors mentioned are, in general, simple “empty” tubular reactors, i.etubular reactors without additional internals such as a static mixer ora stirrer. Owing to the large specific surface area of a tube they areparticularly advantageous with strongly exothermal polymerizations.Thus, for example, continuous suspension polymerization is described inDE-A-2342788 and DE-A-880938. There are also numerous studies whichaddress emulsion polymerization in a tubular reactor, whether it be withlaminar or with turbulent flow. An overview of this work is found inReactors, Kinetics and Catalysis, AIChE Journal, 1994, Vol. 40, 73-87.

One of the main problems in carrying out an emulsion polymerization in atubular reactor is that of the tubes being plugged by coagulate, whichis a particular problem in the turbulent flow domain. Attempts havetherefore been made, by additional measures and special reactionengineering, to prevent coagulate formation, for example with the aid ofpulsed tubular reactors, see the last-mentioned publication, or byoptimizing the reactor dimensions and the material for the tubes, seeACS Symp. Ser. 104, 113 (1979). Patent publications which describetubular reactors for emulsion polymerization are DE-A-26 17 570,DD-A-238 163, DD-A-234 163, DD-A-233 573, DD-A-233 572, DE-A-33 02 251and CZ-A-151 220. FR-A-842 829 and EP-A-633 061 disclose acontinuous-polymerization tubular reactor, in which curved tube segmentsare linked by means of long straight tube segments. Reactors of thistype result in a broad residence time distribution of the unit volumes.

Tubular reactors in most cases are employed in the form of helicallywound reactors, see the two last-mentioned publications. The flowconditions in wound tubular reactors have been particularly wellstudied. The centrifugal forces arising in helically wound tube producea secondary flow perpendicular to the principal flow direction. Saidsecondary flow was first described by Dean and is therefore known as aDean vortex. The Dean vortices in a helically wound tube favor a narrowresidence time distribution of the unit volumes and, in the case oflaminar flow, result in higher heat transfer coefficients and masstransfer coefficients, compared with a straight tube with correspondinglaminar flow. A further improvement is achieved by an abrupt change indirection of the centrifugal forces, see Saxena in AIChE Journal Vol.30, 1984, pp. 363-368 (also compare FIGS. 13 and 14). The change indirection is effected by a 90° kink in the helical winding. By means ofa total of 57 kinks, Saxena et al were able to achieve the narrowestknown residence time distribution in an apparatus with laminar flow. Asa result of the laminar flow, the reaction volume is kept small and onlylow shear forces arise. Even with laminar flow, a high heat transfercoefficient is achieved here.

Notwithstanding all the studies and insights with respect to reactionengineering in connection with disperse systems in tubular reactors,these have not so far been able to find general acceptance in practice.The reason for this is that, on the one hand, the plugging problem intubular reactors has not yet been solved satisfactorily and that, on theother hand, the model described by Saxena et al, having kinked helicalwindings, has decisive drawbacks in practice, such as the difficultfabrication, bulkiness, and the fact that descending turns involvingdescending flow are present, which may have an adverse effect onproductivity. In particular, the formation of gas bubbles of monomers orair can be observed, which results in greater coagulate formation anddisruptions of the progress of the polymerization, and in uneven flowand a change in the residence time of the liquid phase. Additionallythere is the risk that those fractions of the reaction mixture whichhave a higher specific gravity, e.g. the particles present in thedispersion, will settle to a greater extent in the descending turns.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide anapparatus and a process for carrying out liquid-phase chemicalreactions, especially polymerizations, continuously, which have beenimproved with respect to the prior art and permit the reactions to becarried out in a simple manner.

We have found, surprisingly, that this object is achieved if a tubularreactor having a plurality of successive bends having alternatingdirections of curvature is employed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention therefore relates to a curved tubular flow reactorhaving an essentially circular or elliptical cross section, whichcomprises a plurality of preferably directly successive bends havingalternating directions of curvature, a change in the direction ofcurvature taking place at the latest when the distance which the centerof gravity of the cross-sectional area of the tube has traversed fromthe start of a bend is 200 times the diameter of the tube, it beingpossible for the bend to comprise up to three circumvolutions around theaxis of curvature.

The term tube diameter in the case of an elliptical cross section of thereactor refers to the average of the major and the minor axis.

Bends having alternating directions of curvature in this context mean asuccession of curved tube segments, each particular next tube segment(section of the tube between two successive reversals of curvature, e.g.the sections between two axis intersections in FIGS. 1 to 4) pointing ina different direction, preferably the opposite direction to the previousone, i.e. a change takes place with each curved tube segment, preferablya reversal of the direction of curvature. What is achieved by thisdesign of the reactor is as uniform a flow as possible, resulting in theresidence time distribution of the unit volumes being narrowed down.Moreover, the design of the reactor permits the fabrication of windingshaving a spatially particularly favorable, i.e. compact, arrangement, sothat they are particularly suitable for industrial practice (moredetailed discussions of the structures resulting therefrom will followbelow). The reactor according to the invention allows a high Bodensteinnumber to be achieved, which is generally ≧10 and preferably ≧100 andmay be up to 1000 and more. It is a measure of the spread of theresidence time distribution; the higher it is, the narrower and moresymmetric is the residence time distribution.

Preferably, the reversal of the direction of curvature takes place atthe latest when the distance which the center of gravity of thecross-sectional area of the tube has traversed from the start of a bendis 150 times, in particular 100 times, preferably 80 times, particularlypreferably 50 times, 30 times or 25 times the diameter of the tube. Thisdistance generally is at least 5 times the tube diameter, and inparticular it is in the range of from 10 to 150 times, in particularfrom 10 to 100 times, preferably from 10 to 80 times and particularlypreferably from 10 to 50 times, 10 to 40 times or 10 to 30 times thediameter of the tube.

A bend may comprise not only a partial circumvolution, but also one andup to two or three circumvolutions around the axis of curvature (axisthrough the intersection of R₁ and R₂, see FIGS. 1 to 4). The angleswept by the normal vector of the principal flow direction of a benduntil the direction of curvature changes is therefore generally ≦1080°,preferably ≦720° and particularly preferably ≦360°.

The bends are preferably essentially sinusoidal. Sinusoidal bends in thepresent case are to be understood as any type of preferably periodicallyrepeating bends essentially in one plane. Examples of these are thebends shown in FIGS. 1 to 4. Evidently, the ratio R₁/R₂, i.e. the ratioof amplitude to period/4, can be within a wide range. Preferably,however, it is in the range of from 5:1 to 1:50, in particular from 3:1to 1:5 and particuarly preferably it is 1:1.

It is evident, moreover, that not only those bends should be covered inwhich the angle formed between the axis and the tangent in the point ofinflection differs from 90°, but also those in which said angle is 90°,i.e. butted semicircular tube segments, which are particularlypreferred.

The radius of curvature of the curved tube segments is preferably from0.5 to 100 times, preferably from 1 to 80 times, from 2 to 50 or from 2to 20 times the diameter of the cross-sectional area of the tube.

The dimensions of the reactor are generally such that the ratio oflength to diameter is in the range from 100:1 to 1,000,000:1, preferablyfrom 1,000:1 to 100,000.1 or 50,000:1.

Where appropriate, one or more of the curved tube segments may be linkedby straight tube segments. The ratio of straight to curved section oftube is then ≦5, preferably ≦2, especially ≦1 and particularlypreferably ≦0.5 or ≦0.25.

The apparatus may also be composed from a plurality of reactor units,with the option of the reactor, in each unit, having differentgeometries and/or dimensions and/or radii of curvature. For example, inone unit the tube diameter may be increased to achieve a lower flowvelocity, or the radius of curvature may be varied so as to establishspecial product properties.

The cross section of the reactor is preferably circular or elliptical.This also includes modified circular or elliptical cross sections, i.e.cross sections which result from the corners of a square or a rectanglebeing rounded off. In the case of swirl promoter tubes (see below) thebasic shape of the reactor tube is essentially circular or elliptical.

If the cross section is elliptical, the ratio of the semimajor axis tothe semiminor axis is generally in the range of from 5:1 to 1:1,especially in the range of from 2:1 to 1:1.

According to a preferred embodiment, the apparatus is constructed as anascending, as seen from the direction of the incoming flow, andsingle-layer winding around at least two arbors. The arbors may form anangle with respect to one another, but are preferably essentiallyparallel. In the case of a non-self-supporting winding, these arbors maypreferably be implemented by means of pipes or rods which may be roundor angular. The term “winding around at least two arbors” is used hereinonly for illustrative purposes. It is not necessary for the arborsactually to be implemented in the application, e.g. in the form of pipesor rods. In the case of a winding around 2 parallel arbors the result ise.g. the arrangement shown in FIGS. 5, 6 and 9.

If a winding is produced around a plurality of arbors, which arepreferably arranged in a plane, this results in a ribbon-like orwall-like design, as shown in FIGS. 7 and 8.

Finally it is also possible to produce a winding around a plurality ofessentially parallel arbors which pass through the corners of a polygon,especially an equilateral polygon and run perpendicular to the planethereof. The polygon may have an even and preferably an odd number ofcorners, specifically at least 3 or 5. A heptagon has been found to beparticularly advantageous (see FIGS. 10 and 11). A polygonal winding canbe understood as bends along angled arbors (perpendicular to theabovementioned preferably parallel arbors, which have been combined intoa polygon.

The external spacing of the arbors around which the winding is run maybe varied over a wide range. In general it is from 1 to 10 times,preferably from 1 to 5 times and especially from 1 to 3 times thediameter of the reactor tube, the single to double distance beingparticularly preferred.

Additionally, the winding is also defined by the pitch. This in generalis from 1 to 10 times, especially from 1 to 3 times the diameter of thereactor (in the case of the cross section being circular) or of thearbor pointing in the direction of the pitch if the cross section iselliptical.

The abovementioned windings are a particularly favorable arrangement, inspatial terms, and permit compact construction of the apparatusaccording to the invention. They can easily be moved, which provesparticularly advantageous during maintenance. The number of the windingsarranged above one another can be chosen at will, it depends on therequirements in each particular case.

The curved reactor may be made, depending on the requirements of thereaction to be carried out, from metal, a metal alloy, glass or aplastic. There are no restrictions whatsoever in this respect, the tubeneed only be resistant to the reactants and be stable under the reactionconditions. If the reactor consists of metal, it may be made, forexample, from copper or copper alloys, steel or alloy steel of any type.

If the reactor is made of a plastic, preference is given tofluorine-containing plastics, e.g. tetrafluoroethylene, and topolyethylene, polypropylene or poly(vinyl chloride).

The reactor may be made of special section tubes, especially swirlpromoter tubes or fluted tubes or have an internal coating. Depending onthe requirements of the reaction to be carried out, this coating ischosen to be acid-resistant, alkali-resistant or solvent-resistant, etc.Examples of internal coatings are, in particular, fluorine-containingplastics such as tetrafluoroethylene, polyethylene or polypropylene.Additionally, the interior of the tube may have been rendered inert bychemical treatment, e.g. passivated by treatment with nitric acid, havebeen electropolished or mechanically polished.

The reactor according to the invention may, if required, compriseancillary devices or be combined with other equipment. Expediently,means may be provided, at one or more points along the curved reactor,for feeding in chemicals, for example catalysts, reagents, solventsetc., and/or for cleaning, e.g. by making use of pigging systems.

Additionally, the reactor may comprise pulsation means, e.g. pumps, soas to cause the reaction to proceed in a pulsed manner. Moreover, at thestart or at any point along the curved tube a device may be provided bymeans of which, e.g. for separation purposes, gas bubbles of an inertgas such as nitrogen and/or pigs may be introduced. It is furtherpossible for subsegments of the curved reactor, which as a rule do notexceed 10% of the reactor length, to be provided with conventionalmixing elements, for example packings, static or dynamic mixers.

In general, provision is also made for measuring points and samplingdevices along the curved reactor.

The reactor generally also comprises means for heating or cooling themedium flowing through. Suitable for this purpose may be a heatable orcoolable container, which may or may not be subdivided into zones andtotally or partially encloses the tubular reactor so as to control thetemperature in the desired manner. It is also possible to have theheating or cooling medium flow through the pipes around which thetubular reactor is wound. The reactor may also be equipped with a jacketheating or cooling system.

Finally, the reactor may be combined in any which way with otherapparatuses which may be connected upstream and/or in parallel and/ordownstream.

The novel reactor can be used for carrying out chemical reactionscontinuously in the liquid phase. Examples of such chemical reactionsare polymerizations, depolymerizations, oxidation and reductionreactions etc. The reactor is also suitable for the continuouspreparation of mixtures for the systems solid/liquid, liquid/liquid andliquid/gaseous. A relevant example is the mixing of liquids upstream ofspray heads. The reactor can further be used as a heat exchanger forliquids and gas.

The temperature at which the reactor can be operated is limited only bythe materials used.

The reactor is particularly suitable, however, for continuouspolymerization. The present invention therefore also relates to acontinuous polymerization process making use of the novel reactor.Polymerization in this context relates to free-radical, ionic (anionicand cationic) and thermal polymerization of ethylenically unsaturatedmonomers including living polymerization, as well as to polycondensationand polyaddition. It can be carried out in homogeneous or heterogeneousphase under any of the conditions resulting in polymerization. Examplesof polycondensation and polyaddition are the preparation of polyesterand polyurethane. The reactor is preferably suitable for free-radicalpolymerization. This includes solution polymerization, precipitationpolymerization, bulk polymerization and preferably suspensionpolymerization and emulsion polymerization, including mini-, micro- andinverse emulsion polymerization.

How to carry out polymerizations is known, in principle, to thoseskilled in the art. More detailed information on this subject can befound, for example, in Houben-Weyl, Volume XIV, makromolekulare Stoffe,Georg Thieme Verlag, Stuttgart, 1961.

For polymerization purposes it proved particularly expedient to employ aflow characterized by a Reynolds number of 1-10,000, preferably 10-1000,frequently 20-100. The reaction temperature generally is in the range offrom 5 to 250° C., in particular from 5 to 150° C., preferably from 30to 150° C. and particularly preferably from 50 to 100° C. Thepolymerization can be carried out at standard pressure or underpressure. Additionally, a reduced pressure can be set at the end of thetube.

For the purpose of the novel process, use can be made of anyethylenically unsaturated monomers which can be polymerized under theconditions of free-radical polymerization. The components required forthe polymerization can be fed in at the start of the reactor or alongthe reactor, as required. When a copolymer is being prepared, forexample, the infeed of the comonomer(s) together with the main monomer,or of other components, is possible at the start of or along the curvedreactor at various points.

Monomers suitable for the process according to the invention areC₂-C₆-mono- and -diolefins, C₃-C₆-monoethylenically unsaturated mono- ordicarboxylic acids, their salts or mono- or diesters thereof withC₁-C₁₈-alkanols or -diols or amides and N-mono- orN,N-di-C₁-C₁₈-alkylamides or -hydroxyalkylamides thereof,(meth)acrylonitrile, monoethylenically unsaturated sulfonic acids and/ortheir salts, vinylaromatic compounds, vinyl-C₁-C₁₈-alkylethers,vinyl-C₁-C₁₈-alkylesters, N-vinyllactams or vinyl halides and mixturesof various monomers of one type and/or different types.

Examples of suitable monomers are ethylene, propylene, butadiene,acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconicacid, esters of acrylic acid or methacrylic acid with methanol, ethanol,n-propanol, isopropanol, n-butanol, isobutanol, sec-butanol, t-butanol,pentanol, hexanol, 2-ethylhexanol, octanol, decanol, dodecanol orstearyl alcohol, acrylamide, methacrylamide, acrylonitrile,methacrylonitrile, styrene, α-methylstyrene, vinyl ethyl ether, vinylacetate, vinyl propionate, N-vinylpyrrolidone, vinyl chloride,vinylidene chloride, α-phenylstyrene, vinylpyridine, the sodium salt ofvinylsulfonic acid, acrylamidopropanesulfonic acid or their alkali metalsalts or the sodium salt of sulfonated styrene.

If desired, cross-linking agents can be employed such as theabovementioned mono- or diesters of a C₃-C₆-monoethylenicallyunsaturated mono- or dicarboxylic acid with a C₁-C₁₈-alkanol or -diol orN-mono- or N,N-di-C₁-C₁₈-alkylamide, divinylbenzene, silicon-containingcross-linking agent and the like.

To control the molecular weight, conventional regulators can beemployed, for example mercapto compounds such as mercaptoethanol,mercaptopropanol, mercaptobutanol, mercaptoacetic acid,mercaptopropionic acid, mercaptotrimethoxysilane, butylmercaptan andt-dodecylmercaptan. Also suitable are organic halogen compounds such ascarbon tetrachloride or bromotrichloromethane. The amount of regulatoris generally in the range of from 0.01 to 5 wt %, based on the monomersto be polymerized.

All those free-radical polymerization initiators are potentiallysuitable which are able to initiate free-radical polymerization. Thesemay be either peroxides, for example alkali metal peroxodisulfates orazo compounds. Combined systems can likewise be used, which are composedof at least one organic or inorganic reductant and at least one organicor inorganic oxidant, preferably a peroxide and/or hydroperoxide. Theamount of initiators is generally from 0.01 to 5 wt %, in particularfrom 0.1 to 2 wt %, based on the total amount of the monomers to bepolymerized.

The suspension or emulsion polymerization is carried out in the presenceof suitable emulsifiers and protective colloids. These substances arenormally used in amounts up to 10 wt %, preferably from 0.5 to 5 wt %and in particular from 0.5 to 3 wt %, based on the monomers to bepolymerized.

Examples of suitable protective colloids are poly(vinyl alcohol)s,cellulose derivatives or copolymers based on vinylpyrrolidone. Suitableemulsifiers are, in particular, anionic and nonionic emulsifiers such asethoxylated mono-, di- and trialkylphenols, ethoxylates of long-chainalkanols, alkali metal salts and ammonium salts of alkyl sulfates, ofsulfuric acid hemiesters of ethoxylated alkanols and ethoxylatedalkylphenols, of alkylsulfonic acids and of alkylarylsulfonic acids. Adetailed description of protective colloids and emulsifiers is found inHouben-Weyl, Methoden der Organischen Chemie, Volume XIV/1,Makromolekulare Stoffe, Georg-Thieme Verlag, Stuttgart, 1961, pp.192-208 and 411 to 420.

As a rule, the polymerization is carried out by the continuous phasefirst being introduced as the initial charge, i.e. the reactor ischarged with the continuous phase. Then an emulsion of the monomer (ormonomers) in the continuous phase is metered in. The initiator can bemetered in separately or be metered into the emulsion just before thereactor.

The polymerization can also be carried out with the use of a seed latexwhich can be prepared separately or in the first section of the reactor.In the latter case, the monomers to be polymerized are then fed in afterthe seeds have formed.

If desired, a physical or chemical after treatment of the polymerdispersion can be carried out after it leaves the reactor or while stillin the reactor, under customary conditions. This may serve e.g. forremoving or reducing not completely polymerized monomer fractions and/orby-products of the polymerization or impurities of the startingmaterials. Also conceivable is the setting of a desired pH by theaddition of a suitable neutralizing agent, and the addition ofconventional additives such as antioxidants, biocides, cross-linkingagents, colorants and the like.

The process according to the invention is suitable for preparing polymerdispersions comprising a wide polymer mass fraction. Preferably thepolymer mass fraction is in the range from 25 to 75, in particular from50 to 75 vol %. The viscosity of the polymer dispersions can likewisevary over a wide range. In general it is less than 800 mPa.s andpreferably less than 500 mPa.s (determined in accordance with DIN53019). The novel process is particularly suitable for preparing polymerdispersions which, as the main component, comprise acrylates and/orstyrene, in particular those which comprise from 50 to 100 wt % ofesters of acrylic or methacrylic acid with C₁-C₁₂-alkanols or from 50 to100 wt % of styrene with or without copolymerized butadiene.

BRIEF DESCRIPTION OF THE DRAWINGS

The following examples illustrate the invention without limiting it. Inthe figures:

FIGS. 1 to 4 show a schematic representation of sinusoidal bends,

FIG. 5 shows the side view of a reactor according to the invention inthe form of a winding around two rods

FIG. 6 shows a plan view from above of the reactor according to FIG. 5

FIG. 7 shows a reactor according to the invention in the form of awinding around six rods arranged in a plane

FIG. 8 shows a plan view from above of the reactor according to FIG. 7

FIG. 9 shows the side view of a reactor according to the invention inthe form of a winding, modified with respect to FIG. 5, around two rods

FIG. 10 shows a schematic part view of a reactor according to theinvention in the form of a winding around 7 rods arranged at the cornersof an equilateral polygon

FIG. 11 shows a schematic plan view of a winding loop according to FIG.10

FIG. 12 shows a side view of a helical winding according to the priorart

FIG. 13 shows a side view of a helical winding with a 90° kink accordingto the prior art

FIG. 14 shows a side view of a helical winding with four 90° kinksaccording to the prior art

FIG. 15 shows a graph in which the Bodenstein number, determinedexperimentally, according to the prior art, on an apparatus according tothe invention, is plotted against the Reynolds number

FIG. 16 shows a clearer view of FIG. 15 by means of hatching of theranges of the experimental results.

FIG. 17 shows a graph of the residence time distribution in theexperiment according to example 3 in the tubular reactor according tothe invention.

FIG. 18 shows a graph of the torque (as a function of time) at thestirrer used in the comparative experiment according to example 3.

FIGS. 1 to 4 show, by way of example, various options for sinusoidalbends suitable for the curved reactor. It can be seen that the ratio ofR₁ to R₂ (amplitude to period/4) can vary over a wide range.

FIG. 5 shows a reactor according to the invention in the simplestembodiment as a winding. The reactor comprises two rods 1 which areparallel to one another. A tube is wound around these rods so as toresult in a curved reactor 2 having alternating directions of curvature.This can be clearly seen from FIG. 6, i.e. FIG. 6 shows a winding in theform of a prone figure of eight. The distance between the two rods 1 isabout 1.5 times the diameter of the reactor 2. The reactor has an inlet3 and an outlet 4, i.e. the medium in the reactor 2 flows in an upwarddirection.

FIG. 7 shows a further embodiment of a reactor according to theinvention. It comprises 6 rods 1 around which a tube 2 having an inlet 3and an outlet 4 is wound so as to produce an interweave around the rods1, resulting in a reactor in the form of a palisade wall. FIG. 8 showsthat the winding essentially corresponds to the curvature profile shownin FIG. 1.

FIG. 9 shows an alternative embodiment of a winding around two rods 1.The winding runs in such a way that tho radius of curvature of a bendsweeps about 600°.

A further embodiment of the novel reactor can be seen in FIG. 10, in theform of a partial side view. FIG. 11 shows a single winding loop aroundseven rods 1 which are located at the corners of an equilateralheptangle. The rods 1 are wound continuously with a tube 2 so as toproduce a basket-like winding. The flow direction is indicated byarrows. This apparatus is distinguished by a compact arrangement and istherefore particularly suitable for industrial applications.

The center of gravity of the cross-sectional area of the tube is shownby 5 in FIG. 10. The distance which the center of gravity of thecross-sectional area of the tube has traversed from the start of a bendto the change in the direction of curvature is the distance the centerof gravity of the cross-sectional area of the tube traverses between 6and 7 in FIG. 11.

EXAMPLE 1

Tubular reactors comprising the winding configurations shown in FIGS. 5and 9 to 14 were fabricated, FIGS. 12 to 14 representing the prior artas described at the outset. The FIGS. 13 and 14 correspond to thearrangement described by Saxena in AEChE Journal Vol. 30, 1984, pp.363-368, a PVC fabric hose having a length of 50 m, an internal diameterof about 1 cm and a wall thickness of 3 mm being used for this purpose.The tubular reactor was wound, in the manner shown in said figures,around supporting pipes having an external diameter of 10.5 cm and 7.5cm, respectively. The tubular reactor therefore had a radius ofcurvature of about 6.05 cm and 4.55 cm, respectively.

Using these reactors, the residence time density curves were determinedby deionized water being tagged intermittently, using NaCl solution as atracer and by the change of the concentration with time being measuredwith the aid of conductivity measuring cells at the reactor inlet andoutlet. The signals were recorded with the aid of an AD converter card,by means of a personal computer.

From the residence time density curves thus obtained it is possible todetermine the characteristic parameters of the curves by fitting a modelsuch as the dispersion model (Chemical Reaction Engineering, OctavLevenspiel, Wiley & Sons, 1972). In the case of the dispersion model adimensionless characteristic, the dispersion number, is obtained as isthe mean residence time. The inverse of the dispersion number isreferred to as the Bodenstein number, which specifies the ratio ofconvective and conductive mass transfer. To allow a comparison betweenthe various tubular reactors, the Bodenstein number was plotted againstthe Reynolds number (characteristic). The results are shown graphicallyin FIGS. 15 and 16. The assignment of the experimental results should beunderstood as follows: The internal diameter of the support pipe, in cm,forms the prefix, separated from which by a point is the number of thefigure which corresponded to the winding. For example, 7.12 thereforemeans: support pipe internal diameter 7 cm, winding according to FIG.12.

The results show that, surprisingly, a distinctly higher Bodensteinnumber is achieved with the aid of the reactor according to theinvention than with the reactors according to the prior art (FIGS. 12 to14).

EXAMPLE 2

The behavior of the novel apparatus for media having higher viscositiesis shown in the next example. A PVC fabric hose having a length of 50 m,an internal diameter of about 5 cm and a wall thickness of about 5 mmwas wound, in the winding configuration shown in FIG. 11, aroundsupporting pipes having an external diameter of 16 cm. The tubularreactor therefore had a diameter of curvature of about 22 cm. The radiusof the locus of the centers of the supporting pipes was 28 cm. Usingthis reactor, residence time distributions were carried out by solutionsof water and polyethylene glycol being tagged intermittently withidentical solutions which additionally contained KCl. The measurement ofthe change of the correlation over time and the analysis of said changewas carried out in a manner similar to that mentioned in Example 1. Theresults are shown in the table.

Viscosities m²s⁻¹ 1.00 · 10⁻⁶ 2.25 · 10⁻⁶ 5.30 · 10⁻⁶ 9.50 · 10⁻⁶ 1.77 ·10⁻⁶ Re Bo Re Bo Re Bo Re Bo Re Bo 1575 576 723 383 438 341 219 226 126 150  1589 607 817 395 365 319 176 153 111  109  1817 619 944 434 229 219160 128 97 73  717 424 307 252 130 104 87 50 1002 513 234 224 252 250 8250 1226 523 223 224 76 46 1441 583 62 42 1570 613 61 32 1740 684 1981719 2172 778 2201 811 2495 885 2941 1021  Re = Reynolds number Bo =Bodenstein number

EXAMPLE 3

A cross-linking reaction was carried out in a tubular reactor accordingto the invention. The reactor consisted of a PVC fabric hose having alength of 100 m, an internal diameter of 10 mm and an external diameterof 16 mm. The hose was wound, rising steadily, around a structureconsisting of seven core tubes in the form of an equilateral heptangle,each having an external diameter of 75 mm, as shown in FIG. 11. Thecenters of the core tubes were disposed on a circle having a radius of150 mm.

In order to carry out the reaction, a solution (1) consisting of 4 partsof polyvinyl alcohol, 96.16 parts of water and 0.16 parts of glutaricdialdehyde was pumped into the reactor from a supply tank at roomtemperature. 26% strength nitric acid (2) was metered in with a separatepump, which was disposed directly in front of the reactor, and mixed inwith the help of a static mixer. The static mixer consisted of a glasstube having a length of 200 mm and a diameter of 10 mm, charged with 4mm glass Raschig rings. The ratio of volume flows was 169:1. The flowvelocity was determined at the reactor outlet and was (255+/−10) ml/min.In order to measure the velocity at the reactor outlet, samples weretaken and adjusted to pH 10 with 4 molar sodium hydroxide solution andthe reaction was thus terminated. The viscosity of the outlet sampleswas (35+/−5) mm²/s. The viscosity of the polyvinyl alcohol solution (1)was 12 mm²/s.

In order to determine the residence time distribution, 5 ml of 2 molarpotassium thiocyanate solution were injected at once during thecross-linking reaction through a septum. The input signal was receivedby a conductivity measuring cell and stored in a data acquisitiondevice. Samples having a volume of about 40 ml were taken at the reactoroutlet and not quenched. The samples were stored for at least 10 hours,after which time the polymer had reacted to form a gel. Then 5 ml ofpolymer-free solution were taken from the sample. 1 ml of 0.25 molariron(III)chlorid solution was added and a measurement was made using aspectrometer at a wavelength of 400 nm. The residence time density curvewas derived from the magnitude of this signal (FIG. 17). The input andoutput signals were evaluated analogously to the previous example on thebasis of the dispersion model. The Bodenstein number was determined tobe 59.

During the entire experimental period of 3.5 hours, i.e. about sevenaverage residence times of about 1800 s, the reactor was operatedwithout any problem. The desired cross-linking was carried out asplanned. No gelling inside the reactor or formation of gel lumps at thereactor outlet was observed.

In a comparative experiment, solution (1) was introduced as the initialcharge in a stirred vessel for batchwise operation, the torque of thestirrer being measured in order to observe the viscosity, and solution(2) was added batchwise in an amount analogous to the previous example.The polymerisation lead first to a slow, then to a very steep, increasein viscosity, as is evident from the course of the torque in FIG. 18.Once this point has been reached in a reactor, this will lead to theformation of gel particles and possibly to the reactor being clogged upcompletely.

A comparison of FIG. 17 and FIG. 18 shows that the long, trouble-freeoperating time of the tubular reactor results from the hydrodynamicsand, thus, from the geometry of the reactor.

Additionally, the following particular features of the curved tubularflow reactor of the present invention are noted.

Preferably, in the present reactor, a reversal in the direction ofcurvature takes place at the latest when the distance which the centerof gravity of the cross-sectional area of the tube has traversed fromthe start of a bend is 100 times, in particular 70 times, preferably 50times the diameter of tube.

Also preferably, a reversal in the direction of curvature takes placewhen the distance which the center of gravity of the cross-sectionalarea of the tube has traversed from the start of the bend is in therange of from 10 to 200 times, from 10 to 100 times, in particular from10 to 70 times, preferably from 10 to 50 times and especially preferablyfrom 10 to 50 times and especially preferably from 10 to 30 times thediameter of the tube.

Moreover, it is further preferred that the radius of curvature be from0.5 to 100 times, in particular from 1 to 80 times, the diameter of thecross-sectional area of the tube.

In the present reactor, it is preferred, for an elliptical crosssection, that a ratio of semimajor axis to semiminor axis be in a rangeof from 5:1 to 1:1.

Further, it is preferred that in the present reactor the pitch of thewinding be from 2 to 10 times the diameter of the tube (where the tubecross section is circular) or the arbor pointing in the direction of thepitch (where the cross-section of the tube is elliptical).

Additionally, it is generally preferred that the spacing between thetubes or rods be from 1 to 3 times the reactor diameter.

Generally, the reactor is made of metal, metal alloy, glass or plasticand preferably of copper steel, alloy steel, fluorine-containingplastic, polyethylene, polypropylene or poly(vinyl chloride). Morepreferably, the reactor has an inside coating or has been treatedinside.

The reactor is generally fabricated from special section tubes, inparticular, swirl promoter tubes or fluted tubes, and is provided withmeans for introducing or feeding chemicals thereinto.

Generally, the reactor is also provided with pulsation means, means forfeeding in bubbles of inert gas, measuring points for sampling andcleaning, and means for cooling or heating the tube contents.

The reactor of the present invention is generally used for carrying outchemical reactions in the liquid phase, for preparing mixtures and as aheat exchanger.

We claim:
 1. A curved tubular flow reactor, comprising at least one tubehaving an essentially circular or elliptical cross section, which tubecomprises a plurality of curved sections having alternating directionsof curvature, and, optionally, one or more straight sections, whereinthe distance from the start of a curved section to a change in directionof curvature, as measured along the center of gravity of thecross-sectional area of said tube, is no greater than 70 times thediameter of the tube, and wherein the ratio of the lengths of straightsections to curved sections of said tube is ≦5.
 2. The reactor of claim1, wherein at least one of said curved sections comprises up to threecircumvolutions around an axis of curvature.
 3. The reactor of claim 1,wherein said curved sections having alternating directions of curvatureform essentially sinusoidal bends.
 4. The reactor of claim 3, whereinthe ratio of amplitude to period/4 of the sinusoidal bends is 1:2 to1:20.
 5. The reactor as claimed in claim 1, wherein the shape of thecurved sections is a semicircle.
 6. The reactor of claim 1, wherein thetube winds around at least 2 essentially parallel arbors.
 7. The reactorof claim 6, wherein the tube winds around a plurality of essentiallyparallel arbors arranged in one plane.
 8. The reactor of claim 6,wherein the tube winds around essentially parallel arbors runningperpendicularly through the corners of an essentially equilateralpolygon, said polygon having n corners, n being an odd number ≧3.
 9. Thereactor of claim 1, wherein said distance is 50 times the diameter ofthe tube.
 10. The reactor of claim 1, wherein the distance is in therange of from 10 to 70 times the diameter the tube.
 11. The reactor ofclaim 1, wherein the distance range of from 10 to 50 times the diameterthe tube.
 12. The reactor of claim 1, wherein the distance is in therange of from 10 to 30 times the diameter of the tube.
 13. The reactorof claim 1, wherein a radius of curvature of at least one of the curvedsections is from 0.5 to 100 times the diameter of the cross-sectionalarea of the tube.
 14. The reactor of claim 13, wherein the radius ofcurvature is from 1 to 80 times the diameter of the cross-sectional areaof the tube.
 15. The reactor of claim 1, wherein when the tube has anelliptical cross section, the ratio of semimajor axis to semiminor axisin said elliptical cross section is in the range of from 5:1 to 1:1. 16.The reactor of claim 1, wherein the ratio of length to diameter of thetube is in the range of from 100:1 to 1,000,000:1.
 17. The reactor ofclaim 6, wherein a pitch of the winding is from 2 to 10 times thediameter of the tube where the cross section of the tube is circular orthe arbors point in a direction of pitch where the cross section of thetube is elliptical.
 18. The reactor of claim 6, wherein the arbors areformed by tubes or rods (1).
 19. The reactor of claim 18, wherein anexternal spacing between the tubes or rods is from 1 to 3 times thediameter of the tube.
 20. The reactor of claim 1, wherein the tube ismade of metal, metal alloy, glass or plastic.
 21. The reactor of claim20, wherein the tube is made of copper, steel or alloy steel.
 22. Thereactor of claim 20, wherein the tube is made of fluorine-containingplastic, polyethylene, polypropylene or poly(vinyl chloride).
 23. Thereactor of claim 20, wherein the tube is coated or treated on the insidethereof.
 24. The reactor of claim 20, wherein the tube is comprised offluted tubes or swirl promoter tubes.
 25. The reactor of claim 1, whichfurther comprises a means for introducing bubbles of inert gas into saidtube.
 26. The reactor of claim 1, which further comprises a device forheating or cooling a content of the tube.
 27. The reactor of claim 1,wherein the tube has the form of an essentially self-supporting winding.28. The reactor of claim 6, wherein said winding is self supporting. 29.A method of carrying out chemical reactions in liquid phase, ofpreparing mixtures or of carrying out heat exchange, which comprises: a)adding reactants, components of a mixture to be mixed, or a substance toor from which heat is to be exchanged to the reactor of claim 1; and b)carrying out chemical reaction of said reactants, mixing said componentsof said mixture, and exchanging heat to or from said substance.
 30. Aprocess for carrying out chemical reactions in a liquid phase or forpreparing mixtures or for exchanging heat comprising carrying out saidchemical reactions or preparing mixtures or exchanging heat in thereactor of claim
 1. 31. A process for continuous liquid-phasepolymerization, of at least one ethylenically unsaturated monomer, whichcomprises carrying out said process in the reactor of claim
 1. 32. Theprocess of claim 31, which involves a flow which is characterized by aReynolds number in the range of from 1 to 10,000.
 33. The process ofclaim 31, further comprising adding one or more comonomers at a pointalong the reactor to prepare a copolymer.
 34. The process of claim 31,wherein at least one monomer is used which is selected from among mono-or diolefins, C₃-C₁₆-monoethylenically unsaturated mono- or dicarboxylicacids or mono- or diesters thereof with C₁-C₁₈-alkanols or -diols oramides and N-mono- or N,N-di-C₁-C₁₈-alkylamides thereof,(meth)acrylonitrile, vinylaromatic compounds, vinyl-C₁-C₁₈-alkylethers,vinyl-C₁-C₁₈-alkylesters, N-vinyllactams and vinyl halides.
 35. Theprocess as claimed in claim 34, wherein the monomer is selected fromamong ethylene, propylene, butadiene, acrylic acid, methacrylic acid,C₁-C₁₂-alkyl (meth)acrylates, acrylamide, methacrylamide, acrylonitrile,methacrylonitrile, styrene, α-methylstyrene, vinyl ethyl ether, vinylacetate, vinyl propionate, N-vinylpyrrolidone and vinyl chloride. 36.The process of claim 31, wherein a polymerization temperature in therange of from 5 to 250° C. is employed.
 37. The process of claim 31,wherein the polymerization is carried out in an aqueous or organicmedium.